This invention relates to an improvement of a steam valve of a turbine system in a steam electric power generating plant or a nuclear power plant.
Generally, it is required for a turbine of a steam electric power generating plant or a nuclear power plant to reduce thermal stress at the time of starting. For this reason, at the starting of the turbine, a flow quantity of steam generated by a boiler has to be controlled by a by-pass valve disposed within a main steam valve and having a small diameter under a condition in which a steam control valve is fully opened. A valve rod is disposed at a portion below the by-pass valve so as to push it upwardly. With this construction, since steam under a high pressure is regulated by a small lift of the valve rod, the steam flows into the main steam valve at a considerably high speed, and the valve is often subjected to erosion caused by the abrasive action of oxide scale initially contained in the steam. This erosion damages not only the valve itself but also nozzles or vanes of the turbine.
A conventional steam valve of the type described before comprises a main valve, a by-pass valve, a valve rod with its upper end firmly fitted into the lower portion of the by-pass valve, and a valve seat disposed around the by-pass valve. The main valve is disposed below and in contact with the by-pass valve. The by-pass valve is provided at its peripheral wall with a plurality of through holes extending in a direction parallel to the flow direction of the steam.
The valve rod functions to push the by-pass valve away from the valve seat into the position shown in FIG. 1, wherein the through holes 5 are placed in communication with the flow of steam. When the by-pass valve is so positioned, streams of the steam passing through the opposing holes collide with each other at a central portion of a chamber formed in the by-pass valve, thereby decreasing velocity energy of the streams of the steam. The steam then flows into passages provided at the lower portion of the chamber of the by-pass valve and is discharged into a chamber formed in the main valve. Hydrostatic pressure of the steam discharged in the chamber is increased and the steam is then fed towards the turbine through passages.
However, all the streams of the steam passing through the holes formed through the peripheral wall of the by-pass valve do not always collide against the streams of the steam passing through the opposing holes, and thereby causes turbulent flow of the steam in the chamber of the by-pass valve. In addition, the steam from the by-pass valve is throttled twice by two passages, so that it is difficult to feed a large quantity of the steam to the turbine and it is necessary to design the holes so as to have a considerably large cross-sectional area to avoid adverse effects caused by throttling twice the steam. For all of the foregoing reasons, design of an erosion-resistant and an efficient steam valve is difficult.